1. Field of the Invention
The present invention relates to a method of arranging packets in a wireless communication system, and more particularly, to a method of arranging packets complying with the IEEE 802.11ac standard.
2. Description of the Prior Art
Wireless local area network (WLAN) technology is one of the popular wireless communication technologies in the world. In the beginning, WLAN technology is developed for military use, while in recent years, WLAN technology is widely implemented in consumer electronics, e.g. desktop computers, laptop computers, personal digital assistants, etc., to facilitate convenient and high-speed wireless communication. IEEE 802.11 standard is a set of WLAN protocols established by the Institute of Electrical and Electronics Engineers (IEEE).
In details, IEEE 802.11 is composed of more than 20 different standards distinguished from each other by a letter appended to the end of IEEE 802.11. The familiar IEEE 802.11 series are IEEE 802.11a, 802.11b, 802.11g, 802.11n standard and so on. The most difference among each of the IEEE 802.11 series is modulation method and maximum data rate. For example, for modulating signals, IEEE 802.11a/g/n standard utilize orthogonal frequency division multiplexing (OFDM) method, whereas IEEE 802.11b/g utilize direct-sequence spread spectrum (DSSS) method. IEEE 802.11n standard is different from IEEE 802.11a/g standard in adding a multiple-input multiple-output (MIMO) technique and other features that greatly enhance data rate and throughput.
Please refer to FIG. 1, which is a diagram of an IEEE 802.11n packet structure according to the prior art. An IEEE 802.11n packet consists of a preamble portion in the front of a packet and a data portion after the preamble portion. The IEEE 802.11n preamble is of mixed format, backward compatible with IEEE 802.11a/g standard devices, and includes legacy Short Training field (L-STF), legacy Long Training field (L-LTF), legacy Signal field (L-SIG), high-throughput Signal field (HT-SIG), high-throughput Short Training field (HT-STF), and high-throughput Long Training fields (HT-LTF). L-STF is used for start-of-packet detection, automatic gain control (AGC), initial frequency offset estimation, and initial time synchronization. L-LTF is used for further fine frequency offset estimation and time synchronization. L-SIG carries data rate (which modulation and coding scheme is used) and length (amount of data) information. HT-SIG also carries data rate and length information, and is used for packet detection so that the mixed format or the legacy format the transmitted packet uses can be detected. HT-STF is used for automatic gain control. HT-LTF is used for MIMO channel detection. The data portion further includes service field, physical layer convergence procedure (PLCP) service data unit (PSDU) field, tail field and pad field. The service field is used for synchronizing a descrambler to enable estimation of an initial state of a scrambler in the receiver. The PSDU field is used for carrying user data. The tail field is appended after the PSDU field, and the pad field is used for carrying redundant data to fulfill a maximum length of the IEEE 802.11n packet.
For the achievement of a higher quality WLAN transmission, the IEEE committee creates a new generation IEEE 802.11ac standard, which is IEEE 802.11 VHT (Very High Throughput) standard. IEEE 802.11ac uses the OFDM method and MIMO technique the same as IEEE 802.11n as well. Currently, the frame architecture and padding scheme for an IEEE 802.11ac packet structure is not decided yet. Two purposes for defining the frame architecture and padding scheme are as follows.
IEEE 802.11-10/0064r2 discloses a VHT frame padding structure. Please refer to FIG. 2, which is a schematic diagram of an IEEE 802.11ac packet structure according to the prior art. A VHT-SIG field indicates a maximum duration behind the VHT-SIG field for all users, but does not indicate a length information of a media access control (MAC) protocol data unit (PDU), MPDU, for each user. In such a situation, an end-of-file (EOF) flag in the null subframe is appended in the end of the MPDU instead, such that the decoder keeps operating until detecting the EOF flag, which wastes more power in comparison with a decoder with the length information of MPDU in advance. Moreover, a pad field and a tail field are appended sequentially after the EOF flag. The tail field is appended in the end of the packet. The pad field is divided into a MAC pad and a physical (PHY) pad, to be appended between MPDU and the tail field. The PHY pad is less than 8 bits and is appended after the MAC pad. However, the padding mechanism causes the complexity of circuit processing, and is not compatible with IEEE 802.11n standard.
IEEE 802.11-10/0358r0 discloses a VHT frame length indication structure. Please refer to FIG. 3, which is a schematic diagram of an IEEE 802.11ac packet structure according to the prior art. The IEEE 802.11ac packet structure includes two VHT-SIG fields. One VHT-SIG field indicates the common information for all users, such as a maximum duration for all users, while the other VHT-SIG field indicates individual information related to each user, such as a length of a physical layer convergence procedure (PLCP) service data unit (PSDU) field. However, the IEEE 802.11ac packet structure only defines VHT-SIG field clearly, and does not determine how to define the rest fields. Therefore, no padding process is indicated after the PSDU field, which causes a problem for a decoder to process the rest data after the PSDU field.